By grouping statistics into these things we call "countries" we are also "excluding" certain statistics. For instance, why are we ignoring cartel murders just south of the US/Mexican border even though the US drug market is the driving factor?

The parent comment is providing a better, more specific grouping that helps shed light on where murders are actually happening in the US and points us towards where we need to focus our efforts if we want to actually change the murder rate in the US. Inner city interventions will result in the greatest improvements to the murder rate, whereas focusing efforts elsewhere has literally zero chance of bringing the murder rate into line with the rest of the G8 because that's not where the murders are happening.